Coating forming method and coating forming material, and abbrasive coating forming sheet

ABSTRACT

At first, a brazing filler metal sheet is prepared (step S 1 ). The brazing filler metal sheet comprises a brazing filler metal layer, a sticking material layer, and a released paper. The brazing filler metal layer comprises a brazing filler metal. A coating material layer is laminated on the brazing filler metal layer (step S 2 ). The coating material layer comprises a mixture of coating material particles and a binder. As the coating material particles, MCrAlY particles and abrasive particles (cubic boron nitride particles or the like) are used. The coating material layer is then dried (step S 3 ), and the brazing filler metal sheet is cut (step S 4 ), and adhered to a rotor blade (step S 5 ). The rotor blade is heated (step S 6 ), to melt the brazing filler metal. The brazing filler metal gets wet as a liquid phase around the MCrAlY particles, and diffuses due to the heat treatment holding process. A solidified layer is then formed by cooling (step S 7 ). This solidified layer is subjected to blasting (step S 8 ) to allow the cubic boron nitride particles to protrude, thereby formation of the abrasive coating is completed.

TECHNICAL FIELD

[0001] This invention relates to a method of forming an abrasivecoating, an oxidation-resistant coating or the like on, for example, arotor blade, a stator blade, or a shroud in a combustion engine (gasturbine, jet engine, and the like) or a steam turbine, a coatingformation material, an abrasive coating formation sheet, and a rotorblade in a gas turbine on which an abrasive coating or the like isformed by this coating formation method, and a gas turbine using thisrotor blade.

BACKGROUND ART

[0002] In a gas turbine, a predetermined clearance is provided between atip of the rotor blade and the shroud that faces the tip of the rotorblade, so that the tip of the rotor blade and the shroud do not come incontact with each other during operation. If this clearance is toolarge, combustion gas leaks from the pressure surface side to thesuction surface side of the rotor blade, thereby the combustion gas thatcan be used for driving the turbine decreases. As a result, theoperation efficiency of the gas turbine decreases. Therefore, theclearance is set as small as possible, for suppressing the leak of thecombustion gas as much as possible, to improve the performance of thegas turbine.

[0003] However, if the clearance is too small, in the initial stage ofstartup of the gas turbine, the tip of the rotor blade and the shroudmay slide with each other, resulting from thermal expansion of the rotorblade, eccentricity of a turbine rotor, vibrations occurring in thewhole gas turbine, or the like (a so-called initial sliding). When thegas turbine is operated for long time, the shroud exposed to thehigh-temperature gas gradually causes a thermal deformation, thereby thetip of the rotor blade and the shroud may slide with each other (aso-called secondary sliding).

[0004] In general, the shroud comprises a coating as a thermalinsulation or antioxidation on the internal peripheral face thereof. Forexample, a thermal barrier coating (TBC) may be provided for thermalinsulation, or an antioxidant coating consisting of McrAlY may beprovided, where M is one or more of iron, nickel, and cobalt. Thesecoatings often have high hardness, and hence, if the tip of the rotorblade and the internal peripheral face of the shroud slide with eachother, the rotor blade may be largely damaged.

[0005] Japanese Patent Application Laid-Open Nos. 4-218698 and 9-504340,and U.S. Pat. No. 5,702,574 disclose a rotor blade having an abrasivecoating, in which abrasive particles are dispersed in a matrix ofMcrAlY, which is an antioxidant material. In this rotor blade, forexample, cubic boron nitride (CBN) or the like is used as the abrasiveparticles. The cubic boron nitride is a material having high hardness,and hence, if the rotor blade and the internal peripheral face of theshroud slide with each other, the abrasive particles comprising thiscubic boron nitride polish the internal peripheral face of the shroud.As a result, appropriate clearance can be maintained between the rotorblade and the shroud.

[0006] This abrasive coating may be formed as follows. That is, abrasiveparticles are temporarily fixed to the rotor blade body, and a matrix isformed around the abrasive particles by electrodeposition. In otherwords, the matrix is formed by the growth of a deposit. Since the growthof the deposit requires time, this forming method has poor efficiency.Further, the formation of the matrix by the electrodeposition isgenerally expensive. Further, electrodeposition needs large-scaleequipment, and it is difficult to newly build the electrodepositionequipment from a standpoint of environmental protection.

[0007] Japanese Patent Application Laid-Open No. 10-30403 discloses anabrasive coating formation method in which a matrix is formed by athermal spraying method. The thermal spraying method is a method ofallowing a metal layer to grow by injecting a molten metal, and has afeature in that it is highly efficient as compared with theelectrodeposition method. In the thermal spraying method, however, whenabrasive particles are temporarily fixed to the rotor blade body, theelectrodeposition method is used. Therefore, it has the problemsdescribed above, and it is also difficult to accurately control thethickness of the matrix, and large-scale thermal spraying equipment isrequired. When abrasive particles such as cubic boron nitride aredispersed in the metal matrix by the thermal spraying method, since theabrasive particles are buried in the molten metal, it is necessary toremove the molten metal until the abrasive particles are exposed.However, it is difficult to expose the abrasive particles, and hence, itbecomes difficult for the abrasive particles to polish the internalperipheral face of the shroud. Further, the metal matrix may be weldedon the internal peripheral face of the shroud, to damage the rotorblade.

[0008] An antioxidant coating of the TBC or MCrAlY may be formed on theinternal peripheral face of the shroud. These coatings are generallyformed by the thermal spraying method, such as an atmospheric plasmaspray (APS) method, a high velocity oxygen fuel (HVOF) method, a lowpressure plasma spray (LPPS) method, or a detonation gun (D-GUN) method.

[0009] It is an object of the present invention to provide a coatingformation method, a coating formation material, an abrasive coatingformation sheet, a rotor blade in a gas turbine, on which an abrasivecoating or the like is formed by the coating formation method, and a gasturbine using this rotor blade.

DISCLOSURE OF THE INVENTION

[0010] In order to achieve these objects, the coating formation methodaccording to the present invention includes the following steps (1) to(3).

[0011] (1) a lamination step of laminating a brazing filler metal layercomposed mainly of a brazing filler metal and a coating material layercomposed mainly of a coating material, on the surface or the back of anobject to be coated;

[0012] (2) a melting step of heating the laminated brazing filler metallayer and coating material layer to diffuse the coating material and thebrazing filler metal, while allowing the brazing filler metal componentto melt and infiltrate in the coating material; and

[0013] (3) a fixing step of solidifying the molten brazing filler metalto fix it on the object to be coated.

[0014] In the coating formation method according to the presentinvention, a coating is formed by a so-called brazing. This method ischeap as compared with the plating or thermal spraying method, and doesnot require large-scale equipment, and hence there is little limitationon the application site.

[0015] In this case, it is desired that a coating parameter between thebrazing filler metal and the coating material laminated at thelamination step be from 30:70 to 70:30 inclusive, as in the coatingformation method according to the present invention. The brazing fillermetal is reliably melted in the coating material at the melting step, byselecting the volume ratio in this manner.

[0016] As in the coating formation method according to the presentinvention, it is preferred that the brazing filler metal contains boron.Boron diffuses in the coating material at the melting step, to allow thesolidifying point of the coating material to fall. Therefore, even whenthe coating material is heated at a relatively low temperature, thecoating material melts, and once it melts, boron decreases to raise themelting point. As a result, a problem such as remelting hardly occurs inthe actual operation.

[0017] As in the coating formation method according to the nextinvention, the brazing filler metal is preferably selected frommaterials having a melting point lower than the heat treatmenttemperature of the object to be coated. As a result, the melting stepcan be executed at the same time with the heat treatment of the objectto be coated.

[0018] It is also preferred that the coating material layer to be usedis one in which coating material particles diffuse in a binder, as inthe coating formation method according to the next invention. Laminationof the coating material becomes easy by the binder. Since the bindervolatilizes substantially completely at the melting step, it issuppressed that the quality of the coating deteriorates due to thebinder remaining in the coating. If a volatile binder is used, itvolatilizes easily at the melting step. Hence, the quality of thecoating can be further improved, by reducing the quantity of the binderremaining in the coating. As the binder, one that volatilizes at a lowtemperature is preferable, and it is also desired to select the binderhaving a certain degree of strength (rigidity) of the coating materialafter the binder has dried.

[0019] As in the coating formation method according to the nextinvention, it is desired that a mass ratio between the binder and thecoating material particles be from 15:85 to 2:1 inclusive. As a result,formation of the coating material layer becomes easy, and liquiddripping of the brazing filler metal at the melting step can besuppressed.

[0020] One example of a preferable coating material layer includes onecomprising, as main component, MCrAlY particles and cubic boron nitrideparticles. An abrasive coating can be obtained by this coating materiallayer. In this abrasive coating, cubic boron nitride serves as abrasiveparticles, and MCrAlY becomes a matrix to fix the abrasive particles.The MCrAlY matrix also suppresses oxidation of the abrasive particles orthe rotor blade material.

[0021] As in the coating formation method according to the nextinvention, it is desired that the volume ratio between the MCrAlYparticles and the cubic boron nitride particles is from 1:2 to 2:1inclusive, from a standpoint of consistence of improvement in thepolishing ability and reliable fixation of the abrasive particles.

[0022] As in the coating formation method according to the nextinvention, if the abrasive coating is formed at the tip of the rotorblade of a gas turbine, the abrasive coating polishes the internalperipheral face of the opposite shroud, and hence a damage of the rotorblade by adhesion can be prevented.

[0023] In this coating formation method, it is desired to include anexposure step of removing a part of MCrAlY from the surface of the fixedcoating material layer to expose the cubic boron nitride particles, asin the coating formation method according to the next invention.

[0024] The preferable exposure method is blasting, as in the coatingformation method according to the next invention. As in the coatingformation method according to the next invention, it is desired that inthe blasting, an abrasive harder than the MCrAlY particles but softerthan the abrasive particles be used. As a result, since MCrAlY can beremoved efficiently from the formed abrasive coating, the abrasiveparticles can be exposed sufficiently.

[0025] In the blasting, as in the coating formation method according tothe next invention, it is desired that the particle size of the abrasiveis smaller than that of the abrasive particles and smaller than thespace between the abrasive particles. However, if the particle size ismade too small, the abrasive particles attack the holder of the abrasiveparticles to cause a dropout, and hence precautions should be takenregarding this point. As a result, dropout of the abrasive particles canbe suppressed to a minimum, while sufficiently exposing the abrasiveparticles, and hence sufficient polishing performance can be exhibitedfrom the initial stage.

[0026] Other examples of the preferred coating material layer includeone composed mainly of the MCrAlY particles, as in the coating formationmethod according to the next invention. A coating having an oxidationresistance and an intergranular corrosion resistance obtained by thiscoating material layer can be preferably used in various members of agas turbine where high-temperature gas circulates, more specifically, ina rotor blade, a stator blade, and a shroud, as in the coating formationmethod according to the next invention.

[0027] A coating formation coating material according to the nextinvention contains abrasive particles such as cubic boron nitride,Al2O3, SiC, or the like, a metal material having at least an oxidationresistance, and a binder. Since this coating formation coating materialcontains abrasive particles, a metal material, and a binder, the brazingfiller metal is absorbed in the gap produced by volatilization of thebinder, in the heat treatment at the time of coating formation. Thereby,dripping of the brazing filler metal to the surroundings can beconsiderably reduced, and hence the quality (uniformity of the coatingthickness) after forming the coating on the object to be coated can beimproved. As a result, since adjustment of the coating thickness afterforming a coating can be kept to a minimum, the time and energy forcoating formation can be reduced.

[0028] The object to be coated of the present invention includes a rotorblade and a shroud of a gas turbine. Since these objects to be coatedare used in an atmosphere where high-temperature combustion gas isinjected, the life thereof becomes short because of the reducedthickness due to oxidation. However, since the metal material containedin the coating formation coating material according to the presentinvention has an oxidation resistance, oxidation hardly occurs even insuch an atmosphere. Therefore, the abrasive particles can be reliablyheld to demonstrate stable polishing performance, even in long-term usethereof. Further, it has an effect of reducing reduction of thickness ofthe base metal due to oxidation, and hence more stable operation of thegas turbine can be realized.

[0029] In a coating formation coating material according to the nextinvention, in the coating formation coating material, a ratio betweenthe mass of the binder and the mass of the abrasive particles and themetal material is from 15:85 to 2:1 inclusive. As a result, the coatingmaterial layer can be formed easily, and dripping of the brazing fillermetal at the melting step can be suppressed.

[0030] In a coating formation coating material according to the nextinvention, in the coating formation coating material, the metal materialis MCrAlY. Since MCrAlY having an oxidation resistance is used as themetal material for forming a coating, even when a coating is formed onthe rotor blade of a gas turbine used in a high-temperature oxidativeatmosphere, the abrasive particles can be held for long time to maintainthe polishing performance, and to protect the base metal from oxidation.As a result, stable operation of the gas turbine can be realized.

[0031] In a coating formation coating material according to the nextinvention, in the coating formation coating material, the volume ratiobetween the MCrAlY particles and the abrasive particles is from 1:2 to2:1 inclusive. If the ratio of the cubic boron nitride, Al2O3, or SiCused as the abrasive particles is large, the content of MCrAlYdecreases, and hence not only the oxidation resistance decreases, butalso insufficient brazing filler metal easily occurs at the time ofapplication. Further, holding of the abrasive particles becomesinsufficient during brazing, thereby causing a relief of particles. Onthe other hand, if the ratio of MCrAlY is too large, the polishingability of the abrasive coating may be insufficient. From these pointsof view, if the mass ratio is within the range described above, theoccurrence of insufficient brazing filler metal can be prevented, andthe workability can be improved. Further, since the oxidation resistanceof the metal layer that holds the abrasive particles sufficiently ishigh, the particles can be stably held for long time, and dropout of theabrasive particles can be suppressed, thereby enabling reliableoperation of the gas turbine.

[0032] In an abrasive coating formation sheet according to the nextinvention, a brazing filler metal and any one of the coating formationcoating materials described above are laminated. In this abrasivecoating formation sheet, since a binder is contained in the coatingformation coating material, the brazing filler metal is sucked in thespace where the binder volatilizes, in the heat treatment at the time ofcoating formation. As a result, liquid dripping at the time of coatingformation can be considerably reduced, and hence the quality after thecoating has been formed on the object to be coated can be improved.Since the adjustment after coating formation can be kept to a minimum,the time and energy for coating formation can be reduced. This abrasivecoating formation sheet is adhered to the object to be coated, and thenthe abrasive coating can be formed only by heat-treating the object tobe coated, and hence the abrasive coating can be formed very easily, ascompared with the plating or thermal spraying method. Further, if ametal material having an oxidation resistance and an intergranularcorrosion resistance is used as the coating formation coating material,even when the abrasive coating is formed on the rotor blade, the shroudand the like in the gas turbine, which are used in a high-temperatureoxidative atmosphere, dropout of the abrasive particles can besuppressed to thereby maintain stable polishing performance. As aresult, stable operation of the gas turbine can be realized.

[0033] Since the treatment prior to the heat treatment is completed onlyby adhering this abrasive coating formation sheet to the object to becoated, the work becomes very easy. Further, since it is a sheet form,it can be appropriately cut according to the shape of the object to becoated. Therefore, it can easily correspond to objects to be coatedhaving various shapes.

[0034] In an abrasive coating formation sheet according to the nextinvention, in the abrasive coating formation sheet, the coatingparameter between the brazing filler metal and the coating formationcoating material is from 30:70 to 70:30 inclusive. By selecting thevolume ratio, not only the coating formation coating material reliablymelts at the melting step, but also the formed coating becomes strong.

[0035] In an abrasive coating formation sheet according to the nextinvention, in the abrasive coating formation sheet, boron is containedin the brazing filler metal. Since boron is contained, at the meltingstep, this boron diffuses in the coating formation coating material, toallow the solidifying point of the coating formation coating material tofall. Therefore, even when the coating formation coating material isheated at a relatively low temperature, the coating formation coatingmaterial melts. After boron diffuses, since the melting point of thecoating formation coating material increases, the heat resistance of thebrazing filler metal increases. As a result, even when the coatingformation coating material is used in a high-temperature gas, such as inthe rotor blade and the shroud of the gas turbine, the brazing fillermetal can be used without remelting.

[0036] In an abrasive coating formation sheet according to the nextinvention, in the abrasive coating formation sheet, the brazing fillermetal is selected from materials having a melting point lower than theheat treatment temperature of the object to be coated. As a result, themelting step is allowed to progress at the same time with the heattreatment of the object to be coated.

[0037] In an abrasive coating formation sheet according to the nextinvention, in the abrasive coating formation sheet, an adhesive layer isformed on the brazing filler metal. Therefore, so long as the abrasivecoating formation sheet is prepared, the treatment prior to the heattreatment is completed only by adhering the abrasive coating formationsheet on the object to be coated, without requiring pasting and waitingfor drying of the paste. As a result, time and energy for coatingformation can be reduced.

[0038] In a rotor blade of a gas turbine according to the nextinvention, a coating is formed at the tip thereof by any one of thecoating formation methods. Therefore, the abrasive coating can be formedvery easily, as compared with the plating or thermal spraying method. Asa result, the time required for coating formation can be considerablyreduced, as compared with the coating formation method described above,and the production cost thereof can be reduced.

[0039] In a rotor blade of a gas turbine according to the nextinvention, any one of the abrasive coating formation sheets is adheredto the tip thereof. Therefore, the abrasive coating can be formed onlyby performing the necessary heat treatment on the rotor blade, and hencethe abrasive coating can be formed very easily, as compared with theplating or thermal spraying method. As a result, the time required forcoating formation can be considerably reduced, as compared with thecoating formation method described above, and the production costthereof can be reduced.

[0040] A gas turbine according to the next invention comprises: acompressor that compresses air to produce combustion air; a combustorthat allows the combustion air produced by the compressor to react witha fuel, to generate a high-temperature combustion gas; and a turbinehaving a rotor blade driven by the combustion gas injected from thecombustor to the rotor blade.

[0041] Therefore, so long as the heat treatment equipment is provided,the abrasive coating can be formed easily, and hence the equipment forcoating formation becomes simple, as compared with the plating orthermal spraying method. Hence, even when there is no plating equipmentnear the gas turbine plant, the abrasive coating can be easily formed,if only a heating furnace used for the heat treatment is provided.Hence, the abrasive coating can be formed again on the rotor blade orthe like, on the site. As a result, even if the abrasive coating isdamaged, repair is easy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a flowchart that shows the coating formation methodaccording to one embodiment of the present invention;

[0043]FIG. 2 is a block diagram for explaining various steps in thecoating formation method in FIG. 1;

[0044]FIG. 3 is a perspective view that shows a rotor blade, on which anabrasive coating is formed by the forming method in FIG. 1;

[0045]FIG. 4 is an enlarged cross section that shows a part of the rotorblade shown in FIG. 3; and

[0046]FIG. 5 shows a gas turbine having a gas turbine rotor blade, atthe tip of which an abrasive coating is formed by the coating formationmethod according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0047] An exemplary embodiment of the present invention are explained indetail below with reference to the accompanying drawings, however, thepresent invention is by no means limited only to this embodiment. Thecomponents in the embodiment include one that can be assumed easily bythose skilled in the art, or substantially the same one.

[0048]FIG. 1 is a flowchart of the coating formation method according toone embodiment of the present invention. This coating formation methodis applied to a case in which a relatively simple apparatus (forexample, a high vacuum heating furnace) is used, to form an abrasivecoating at the tip of the rotor blade of a gas turbine. In this coatingformation method, at first, a brazing filler metal sheet is prepared(step S1). FIG. 2(a) is an enlarged cross section of a part of thisbrazing filler metal sheet, which is comprehensively denoted byreference sign 1. This brazing filler metal sheet 1 comprises a brazingfiller metal layer 3 on the upper side in the figure, an intermediatesticking material layer 5 as an adhesive layer, and a released paper 7on the lower side. Needless to say, the brazing filler metal layer 3comprises a brazing filler metal. If only the sticking material layer 5and the lower side released paper 7 are provided, the released paper 7is peeled off, and the brazing filler metal sheet 1 has only to beadhered on the object to be coated, and hence the work becomes veryeasy. The thickness of the brazing filler metal layer 3 is generallyfrom 0.05 mm to 1.00 mm. The brazing filler metal layer 3 may be asingle sheet or a bundle of two or three sheets. The intermediatesticking material layer 5, being an adhesive layer, and the lowerreleased paper 7 may be provided according to need. When theintermediate sticking material layer 5 and the lower released paper 7are not provided, the sheet 1 can be adhered on the object to be coatedby pasting or the like, using a binder as a paste.

[0049] A preferable brazing filler metal includes one containing boron(B) of from about 2.75 to 3.50% by mass and composed mainly of nickel(Ni). This brazing filler metal generally contains chromium (Cr) of fromabout 6 to 8% by mass, silicon (Si) of from about 4 to 5% by mass, andiron (Fe) of from about 2.5 to 3.5% by mass. The brazing filler metalsheet 1 is preferably one that is not hardened with lapse of time, and aspecific example of the brazing filler metal sheet 1 includes BNi-2 (JISStandard), or the like.

[0050] The brazing filler metal sheet 1 is available in the market inthe form such that the sticking material layer 5 and the released paper7 are laminated on the brazing filler metal layer 3 in advance, and thebrazing filler metal contains 83% by mass of nickel, 7% by mass ofchromium, 3% by mass of boron, 4% by mass of silicon, and 3% by mass ofiron.

[0051] Subsequently, a coating material layer 9 shown in FIG. 2(b) isformed on the brazing filler metal sheet 1 (step S2). It is alsopossible to form the coating material layer 9 and the sheet 1separately, cut them from where they were formed, and later stick themtogether by a binder or the like. In order to prevent cracking such ascut in multiple steps, these are formed as a bilayer with the softBNi-2, to thereby improve the sectility of the whole sheet.

[0052] The coating material layer 9 is formed by coating a mixture ofcoating material particles and a binder 11 on the surface of the brazingfiller metal layer 3. At first, the mixture of the coating materialparticles and the binder 11 is poured on the brazing filler metal layer3. Excessive mixture is scraped off, while spreading out the mixture inthe form of sheet by a blade or the like, and coated in a predeterminedthickness, taking the shrinkage allowance into consideration, when themixture is dried. The coating material 9 is dried after coating (stepS3), and generally air-dried for about one day. Because of the drying,the binder 11 volatilizes to some extent, whereby the thickness of thecoating material layer 9 decreases.

[0053] The predetermined thickness of the coating material layer 9 aftercoating may be about from 0.10 to 1.00 mm, so that the thickness of thecoating material layer 9 after drying becomes not larger than thecoating thickness of the brazing filler metal sheet 1, as a standard.Therefore, it is preferred to appropriately change the thickness of thecoating material layer 9 after coating the mixture, depending on themixing ratio of the binder 11 and the component ratio of the coatingmaterial layer 9.

[0054] MCrAlY particles 13, being a metal material having an oxidationresistance and an intergranular corrosion resistance, are used ascoating material particles, and cubic boron nitride particles 15 areused as the abrasive particles. Hereinafter, it is assumed that when itis simply referred to as coating material particles, this indicates bothof these particles. The binder is mixed with the coating materialparticles and the abrasive particles, to become the coating formationcoating material that forms the coating material layer 9.

[0055] MCrAlY is an alloy mainly composed of iron (Fe), nickel (Ni) orcobalt (Co), a chromium (Cr), aluminum (Al), and yttrium (Y), having theoxidation resistance and the intergranular corrosion resistance. It ispreferred to increase the content of Cr and Al, to improve theintergranular corrosion resistance and the oxidation resistance, takingit into consideration that MCrAlY is diluted by the Ni brazing fillermetal, after forming a coating. However, if the quantity of these,particularly, the quantity of Al is too much, the brazing propertydeteriorates, and hence precautions should be taken. In order to improvethe oxidation resistance, the intergranular corrosion resistance, andthe brazing property, Ta, Re, Hf, Si or the like can be added, inaddition to Cr and Al.

[0056] It is necessary to bring impurities with respect to the brazingproperty, such as O and N, close to zero infinitely on the surface ofthe MCrAlY particles. As the MCrAlY, it is preferred to use one havingthe particle size in the range of from 10 to 100 μm at random, in orderto increase the filling rate. However, if the particle size is toosmall, the surface area becomes too large, thereby disadvantageouslyincreasing the amount of impurities such as O and N.

[0057] On the other hand, as the cubic boron nitride particles 15, oneput on the market by General Electric company, De Beers industrialDiamonds, Showa Denko K.K., Sumitomo Electric Industries, Ltd. and thelike can be used. The cubic boron nitride is classified in singlecrystal and polycrystal, and high purity products exist. It is possibleto use the right one in the right place, but it becomes clear that oneobtained by coating cubic boron nitride with TiN or the like hasexcellent brazing property. Coating on the cubic boron nitride improvesthe wettability between the cubic boron nitride and the brazing fillermetal, and hence the cubic boron nitride particles 15 can besufficiently buried in the brazing filler metal. Thereby, dropout of thecubic boron nitride particles 15 can be suppressed. As a result, the TBClayer or the like on the shroud can be shaved off stably, therebypreventing welding between the tip of the rotor blade and the shroud,and enabling highly reliable operation.

[0058] Further, one obtained by coating the cubic boron nitride with Coor Ni, or one obtained by coating the cubic boron nitride with TiN or aTi compound may be used. It is desired to appropriately select thesedepending on the kinds of the MCrAlY particles forming the coatingmaterial layer 9.

[0059] For example, Al₂O₃, SiC, and the like may be used as the abrasiveparticles, instead of the cubic boron nitride particles 15. When Al₂O₃,SiC, or the like is used in order to improve the wettability with thebrazing filler metal, it is preferred to use Al₂O₃ or SiC applied withcoating. From a standpoint of improving the membrane-making with respectto Al₂O₃ and TiN, and the wettability with respect to the MCrAlYmaterial, when Al₂O₃ is used as the abrasive particles, Co, Cr, Ni andthe like can be mentioned as these coating materials. When SiC is usedas the abrasive particles, AlN, TiN, Al₂O₃ and the like can be mentionedas the coating material used for suppressing the reaction with SiC andCr during brazing.

[0060] The volume ratio of the volume V_(M) of the MCrAlY particles 13to the volume V_(C) of the cubic boron nitride particles 15,V_(M):V_(C), is preferably from 30:70 to 70:30 inclusive. If the ratioof the cubic boron nitride particles 15 is large, percentage of voids inthe brazing filler metal sheet 1 increases, and the amount of the binderalso increases. As a result, insufficient brazing filler metal anddeformation likely occur. If the percentage of the cubic boron nitrideparticles 15 exceeds 70%, that is, the volume ratio V_(M):V_(C) issmaller than 30:70, the density of the cubic boron nitride particles 15is too large, and electric discharge machining from above the coatingbecomes difficult. Since the oxidation resistance also decreases, thereis the possibility that the holding power for the cubic boron nitrideparticles 15 decreases to cause dropout of the cubic boron nitrideparticles 15. From this point of view, it is preferred to set the volumeratio V_(M):V_(C) to not smaller than 1:2, and it is particularlypreferable that the percentage of the cubic boron nitride particles 15be not larger than 60%, that is, the volume ratio V_(M):V_(C) be notsmaller than 40:60, from a standpoint of cost. On the other hand, if thevolume ratio V_(M):V_(C) exceeds 70:30, the polishing ability of theabrasive coating may be insufficient. From this point of view, it ismore preferable to have the volume ratio V_(M):V_(C) of not larger than2:1, and particularly preferable to have the volume ratio V_(M):V_(C) ofnot larger than 60:40. Therefore, the most preferable range of thevolume ratio V_(M):V_(C) is from 40:60 to 60:40 inclusive. At the timeof actual operation, since the specific gravities (densities) of therespective materials are known, the sheet 1 is prepared under masscontrol.

[0061] It is also known that the cubic boron nitride particle 15 hashigh hardness at high temperatures and excellent machinability, butdisappears in a short period of time in a high-temperature oxidativeatmosphere. Therefore, it is necessary to use it by mixing it with SiC,Al₂O₃, and the like having excellent long term stability. These volumeratios are also applicable, when Al₂O₃, TiN and the like are used,instead of the cubic boron nitride particles 15, or together with thecubic boron nitride particles 15.

[0062] Various kinds of binders can be used as the binder 11, but it isparticularly preferable to use one that volatilizes at a lowtemperature. The volatile binder 11 volatilizes in the dry step and themelting step described later in detail, and hence hardly remains in theabrasive coating. Therefore, it does not adversely affect the quality ofthe abrasive coating. Further, after the volatile binder 11 volatilizes,a gap is formed. In the melting step described later, since the brazingfiller metal is absorbed in this gap due to the capillary phenomenon,liquid dripping can be considerably reduced. As a result, degradation ofthe rotor blade due to the liquid dripping is suppressed, and atreatment for the liquid dripping (mainly application of stop-off) ishardly required, and hence time and energy for application can beimproved.

[0063] Organic binders can be preferably used as the preferable volatilebinder 11, and particularly, a cellulose binder is more preferablebecause of having excellent flowability of the brazing filler metal.When a binder in which a plasticizer is added to the binder is used,flexibility is added to the abrasive coating formation sheet 1 adescribed later, thereby enabling improvement in the workability, suchthat the sheet becomes easy to cut, or the like, which is preferable.

[0064] The mass ratio m_(B):m_(C), of the mass ratio m_(B) of thevolatile binder 11 to the mass ratio m_(C) of the coating materialparticles 13 and 15 is preferably from 15:85 to 2:1 inclusive. If themass ratio m_(B):m_(C) is less than the lowest limit of the above range,there is the possibility that the coating of the mixture of the coatingmaterial particles 13 and 15 and the binder 11 becomes difficult. Fromthis point of view, it is more preferable that the mass ratiom_(B):m_(C) be not smaller than 20:80 (1:4), and most preferable thatthe mass ratio m_(B):m_(C) be not smaller than 1:2. On the other hand,if the mass ratio m_(B):m_(C) exceeds the upper limit of the aboverange, liquid dripping likely occurs at the time of heat treatment ofthe object to be coated. From this point of view, it is more preferablethat the mass ratio m_(B):m_(C) be not larger than 60:40, and mostpreferably, not larger than 40:60. Therefore, the preferable range ofthe mass ratio m_(B):m_(C) of the volatile binder 11 to the coatingmaterial particles 13 and 15 is from 20:80 to 40:60 inclusive.

[0065] The coating parameter between the brazing filler metal and thecoating material is preferably from 30:70 to 70:30 inclusive. If thiscoating parameter is less than the lowest limit of the above range, thecoating material does not infiltrate in the brazing filler metal at themelting step, thereby easily causing insufficient brazing filler metal.From this point of view, it is particularly preferable that the coatingparameter be not smaller than 60:40. After the brazing filler metalsheet 1 has been formed, the sheet 1 is dried at a normal temperature,in order to facilitate the cutting operation, and volatilize theexcessive binder. It is desired to dry the sheet for the entire day orlonger in a thermostatic chamber, if possible, in which the temperatureand humidity are controlled.

[0066] The brazing filler metal sheet 1 on which the coating materiallayer 9 is laminated (hereinafter referred to as an abrasive coatingformation sheet 1 a) is cut into a predetermined shape and size (stepS4). The cutting means is not particularly limited, but since theabrasive coating formation sheet 1 a has high brittleness, it ispreferred to use a stencil and a ultrasonic cutter. The released paper 7is peeled off from the abrasive coating formation sheet 1 a cut into thepredetermined shape and size, and adhered on the tip of the rotor blade,being the object to be coated (step S5).

[0067] Since the abrasive coating formation sheet 1 a is cut into ablade shape, the majority thereof remains as unused waste pieces. Sinceexpensive cubic boron nitride is contained in the abrasive coatingformation sheet 1 a, it is necessary to recover the cubic boron nitride.However, a coating for improving the brazing property is applied on thesurface of the cubic boron nitride, and hence it is important to recoverthe cubic boron nitride without damaging this coating. Therefore, inrecovering the cubic boron nitride, it is possible to recover only thecubic boron nitride by boiling and soaking the waste pieces of theabrasive coating formation sheet 1 a in an NaOH solution having aconcentration of about 10% for about 1 to 5 hours to dissolve thebinder, subjecting it to ultrasonic cleaning in pure water, and thenfiltering, washing with pure water, classifying, and drying. Here,ultrasonic cleaning in pure water is performed for about 10 to 30minutes, for example three times, or drying is performed for about onehour, at for example 120° C.

[0068] Prior to attaching the abrasive coating formation sheet 1 a, itis preferred to apply pretreatment, such as blasting, or cleaning by asolvent such as trichloroethylene, acetone or the like, to the tip ofthe rotor blade. It is because the tip of the rotor blade, on which thecoating is formed, becomes rough due to the pretreatment, and oils andfats in the coating formed portion is removed, and as a result,adherence between the coating and the tip of the rotor blade becomesexcellent.

[0069] A hole for the cooling medium such as cooling air and coolingsteam to gush out from the internal cooling passage may be provided atthe tip of the rotor blade. Therefore, if this hole is closed when theabrasive coating formation sheet 1 a is attached to the tip of the rotorblade, the cooling medium cannot gush out during the operation of thegas turbine, and hence cooling of the rotor blade may be insufficient.Therefore, the abrasive coating formation sheet 1 a is attached,avoiding the portion of the hole, from which the cooling medium gushesout. However, if the diameter of the hole is small and there are manyholes, it is difficult to avoid these holes. Further, prior to the heattreatment, the abrasive coating formation sheet 1 a is easily cut, andhence it is difficult to ensure the hole in this sheet before attachingthe sheet. Therefore, punching is possible after attaching the sheet, byelectric discharge machining or the like. Punching by the electricdischarge machining is possible, regardless of before or after the cubicboron nitride particles are exposed.

[0070] The abrasive coating formation sheet 1 a is then heated togetherwith the rotor blade body (step S6). A vacuum heating furnace isnormally used for heating. The heating conditions are determined, takinginto consideration the material of the rotor blade body and the kind ofthe brazing filler metal. For example, when the material of the rotorblade body is a base metal of the rotor blade (Ni group super alloy orthe like), and the BNi-2 is used as the sheet 1 used for the abrasivecoating formation sheet 1 a, at first, the temperature of the vacuumheating furnace is raised from a room temperature to about 600° C. over10 hours or more. Since the abrasive coating formation sheet 1 a isheated, taking long time, to positively volatilize the binder 11 in theabrasive coating formation sheet 1 a at a low temperature, the componentin the binder 11 that is likely to expand by heat does not remain at ahigh temperature. As a result, lines due to thermal expansion do notoccur, and hence the quality of the formed abrasive coating can beimproved. It is desired that the degree of vacuum at this time be higherthan 10⁻⁵ torr. Subsequently, the temperature is raised up to 1000° C.or higher for about 2 hours, and the abrasive coating formation sheet 1a is held in this state for a required period of time. As a result, notonly almost all the binder 11 volatilizes from the coating materiallayer 9, but also a gap is created in the coating material layer 9 afterthe binder 11 has volatilized.

[0071] Since the melting point of the brazing filler metal is about1000° C., the brazing filler metal melts due to heating at 1000° C. orhigher. The brazing filler metal that becomes liquid form due to thisheating, infiltrates into the gap in the coating material layer 9 due tothe capillary phenomenon, and absorbed in this gap. Boron, being thebrazing filler metal component, also diffuses in the MCrAlY particles 13in the coating material layer 9. Since boron lowers the solidifyingpoint of MCrAlY, MCrAlY becomes a half melted state, and easy to diffusein the surrounding brazing filler metal.

[0072] Subsequently, the inside of the vacuum heating furnace is cooledto 500° C. or lower by introducing an argon gas or a nitrogen gas (stepS7). Thereby, the strength required for the Ni alloy, being the basemetal, can be obtained, and as shown in FIG. 2(c), a solidified layer 21is formed, in which the cubic boron nitride particles 15 are dispersedin the MCrAlY matrix 19. Since boron disappears to some extent byholding the sheet at a controlled temperature of 1000° C. or higher, themelting point of the matrix 19 rises to a temperature at which there isno practical problem. By this heat history, the heat treatment(stabilizing treatment) required for increasing the strength of therotor blade is executed. In other words, melting of the coating materialand the heat treatment of the rotor blade are completed at the same timeat the melting step, by selecting a brazing filler metal having amelting point lower than the heat treatment temperature of the rotorblade.

[0073] Generally, cubic boron nitride has a specific gravity lighterthan that of the brazing filler metal, if the both materials are mixedbeforehand, the cubic boron nitride particles 15 float in the surfacelayer in the liquid brazing filler metal, thereby causing unequaldispersion of the cubic boron nitride particles 15 in the solidifiedlayer 21. Further, liquid dripping of the melted brazing filler metaleasily occurs. In the preferred embodiment of the present invention, thebrazing filler metal layer 3 and the coating material layer 9 aresequentially laminated on the tip 17 of the rotor blade, to mix these bythe capillary phenomenon. Therefore, since being held by MCrAlY in thecoating material layer 9, the cubic boron nitride particles 15 does notfloat up, thereby dispersion of the cubic boron nitride particles 15becomes uniform, and liquid dripping of the brazing filler metal can besuppressed.

[0074] Subsequently, blasting is applied to the solidified layer 21(step S8). In the blasting, abrasive blasting particles are sprayed ontothe surface of the matrix 19. By this blasting, as shown in FIG. 2(d),the portion towards the surface of the matrix 19 is removed. Since thecubic boron nitride particles 15 are hardly removed by the blastingaccording to the present invention, the cubic boron nitride particles 15protrude from the matrix 19 (a so-called “exposed”). In this manner, theabrasive coating 23 is completed. In FIG. 2(d), the boundary between theabrasive coating 23 and the tip 17 of the rotor blade is clearly drawn,but in the actual rotor blade, the boundary between these becomesambiguous due to the dispersion at the time of heating.

[0075] In order to remove the portion towards the surface of the matrix19 by blasting prior to the cubic boron nitride particles 15, it ispreferred to use the abrasive blasting particles having hardness lowerthan that of the cubic boron nitride particles 15 but higher than thatof the matrix 19. In other words, if it is assumed that the Vickershardness of the matrix 19 is H1, the Vickers hardness of the cubic boronnitride particles 15 is H2, and the Vickers hardness of the abrasiveblasting particles used for the blasting is H3, it is preferred that H1,H2, and H3 satisfy the relation expressed by the following expression(I):

H1<H3<H2  (1).

[0076] When MCrAlY is used for the cubic boron nitride particles 15 andthe matrix 19, for example, Al₂O₃ particles can be used as the abrasiveblasting particles.

[0077] If the diameter of the abrasive blasting particles is too large,the exposure of the cubic boron nitride particles 15 becomesinsufficient. On the other hand, if the diameter of the abrasiveblasting particles is too small, exposure at the base where the cubicboron nitride particles 15 are held becomes too much, thereby the cubicboron nitride particles 15 drop out from the coating material layer 9.Therefore, it is preferred to use abrasive blasting particles having asize smaller than the space between the cubic boron nitride particles15, and a size such that the abrasive blasting particles does not attackthe base where the cubic boron nitride particles 15 are held. In thisexample, micro blasting using Al₂O₃ particles having a mean particlesize of 50 μm is used, but it is desired to appropriately select thediameter of the abrasive blasting particles to be used, based on theparticle size of the cubic boron nitride particles 15 and spacestherebetween. For example, when the space between the cubic boronnitride particles 15 is large and the surface is rough, it is preferredto use a larger abrasive blasting particle. Further, in the abrasivecoating, when Al₂O₃ and SiC are used in the abrasive particles, it ispreferred to use ZrO₂, glass beads, and the like as the abrasiveblasting particle.

[0078]FIG. 3 is a perspective view of a rotor blade 25, on which anabrasive coating 23 is formed by the forming method in FIG. 1. The rotorblade 25 comprises a body 27 and a protruding portion 29 extending fromthe end of the body 27, and the abrasive coating 23 is coated on theupper face of the protruding portion 29, being the tip of the rotorblade. Though not shown, the internal peripheral face of the shroud islocated, facing the abrasive coating 23, in the gas turbine. When therotor blade 25 and the shroud slide with each other, the internalperipheral face of the shroud is polished by the abrasive coating 23. Arotor blade having no protruding portion 29 may exist, but in this case,the abrasive coating can be formed at the tip of the rotor blade.

[0079] The mean particle size of the cubic boron nitride particles 15 ispreferably from about 50 to 200 μm. If the mean particle size is lessthan 50 μm, the polishing ability of the abrasive coating 23 may beinsufficient. From this point of view, it is particularly preferablethat the mean particle size be not smaller than 80 μm. On the otherhand, if the mean particle size exceeds 200 μm, not only the coatingthickness of the abrasive coating 23 becomes too large, but also theoxidation resistance of the abrasive coating 23 becomes insufficient.From this point of view, in the case of the rotor blade of the gasturbine, it is particularly preferable that the mean particle size benot larger than 170 μm. Therefore, the most preferable range of the meanparticle size is from 80 to 170 μm.

[0080]FIG. 4 is an enlarged cross section of a part of the rotor blade25 shown in FIG. 3. As described above, the cubic boron nitrideparticles 15 protrude from the matrix 19. In this figure, what is shownby two arrows p is a protrusion size of the cubic boron nitride particle15. When it is assumed that the mean particle size of the cubic boronnitride particles 15 is D, and a mean value of the protrusion size p inall cubic boron nitride particles 15 protruding from the matrix 19 (thatis, mean protrusion size) is P, the ratio of the mean protrusion size Pto the mean particle size D is preferably from 25% to 70% inclusive. Ifthis ratio is less than 25%, the polishing ability of the abrasivecoating 23 may be insufficient. From this point of view, it is morepreferable that the ratio be not smaller than 30%. On the contrary, ifthis ratio exceeds 70%, the cubic boron nitride particles 15 may oftendrop out from the matrix 19. From this point of view, it is morepreferable that the ratio be not larger than 60%. Therefore, the mostpreferable range of the ratio is from 30 to 60%.

[0081] It is preferred that the thickness of the matrix 19 (the portionindicated by the duplex arrow T in FIG. 4) be not smaller than 50 μm. Ifthe thickness of the matrix 19 is less than 50 μm, not only holding ofthe cubic boron nitride particles 15 in the abrasive coating 23 becomesinsufficient, but also the distribution of the cubic boron nitrideparticles 15 cannot be arranged in a form of shark teeth, and hence thelong-time high-temperature durability of the abrasive coating 23decreases.

[0082] In this invention, the abrasive coating formation sheet 1 a,being a brazing filler metal sheet 1 laminated with the coating materiallayer 9, is used. This is cut into a predetermined shape and attached tothe object to be coated, and then heat treatment is executed withrespect to the object to be coated, to thereby form the abrasive coating23 (see FIG. 3) on the object to be coated. Subsequently, the abrasivecoating 23 is subjected to the blasting, to expose the cubic boronnitride particles 15, so that the abrasive particles protrude from theabrasive coating 23. After being manufactured in this manner, theabrasive coating formation sheet 1 a is cut into a predetermined shapeand attached to the object to be coated. Thereafter, the abrasivecoating can be formed only by applying the necessary heat treatment tothe object to be coated. Since blasting is used for exposing theabrasive particles, the abrasive particles can be allowed to protrudeeasily. As a result, the abrasive coating can be formed very easily, ascompared with the conventional plating or thermal spraying method.

[0083] For example, when the abrasive coating is to be formed at the tipof the rotor blade of a gas turbine, according to the coating formationmethod of the present invention, the application cost can be suppressedto from 1:3 to 1:4 as that of the conventional plating method. Further,the time required for the application can be shortened to less than 1:3.Since considerable effect of decreasing the application cost andreducing the application period can be obtained, it is very useful whenthe abrasive coating is formed on a large number of rotor blades.Further, since large-scale equipment as in the plating method is notnecessary, the cost required for the investment in plant and equipmentcan be reduced. Since wastewater from plating is not generated as in theplating method, environmental burden can be considerably reduced.

[0084] Further, if heating equipment such as the vacuum heating furnaceis prepared, the abrasive coating can be formed only by supplying theabrasive coating formation sheet 1 a. Therefore, the heat treatment andthe sheet preparation are not necessarily carried out at the same place.Hence, the freedom in application can be increased. For example, even ina gas turbine plant installed in a location where the applicationfacility does not exist in the vicinity thereof, if only the heatingequipment is provided and the abrasive coating formation sheet 1 a isregularly supplied, recoating and the like can be performed on the site.

[0085] In the explanation, an example in which cubic boron nitride andMCrAlY are used as the coating material is shown, but only MCrAlY may beused as the coating material. In this case, the obtained coating is anoxidation resistant coating. This oxidation resistant coating issuitable for the rotor blade, the stator blade, or the shroud of a gasturbine.

[0086]FIG. 5 shows a gas turbine having a gas turbine rotor blade, atthe tip of which an abrasive coating is formed by the coating formationmethod according to the present invention. The air taken in from an airintake 50 is compressed by a compressor 51 to become high temperatureand high pressure compressed air, and is fed to a combustor 52. Thecombustor 52 supplies a gas fuel such as natural gas or the like, or aliquid fuel such as gas oil, light fuel oil or the like to thecompressed air, to burn the fuel, to thereby generate high temperatureand high pressure combustion gas. This high temperature and highpressure combustion gas is guided to a combustor tail pipe 53, andinjected to a turbine 54.

[0087] The turbine 54 comprises a rotor blade 25 (see FIG. 3), at thetip of which an abrasive coating is formed by the coating formationmethod according to the present invention. The rotor blade 25 has acoating according to the present invention formed at the tip thereof.When the operation of a gas turbine 100 is started, a so-called initialsliding occurs due to thermal expansion of the rotor blade, and the tipof the rotor blade 25 may come in contact with the internal wall of ashroud 55. When a certain period of time has passed since starting theoperation, the tip of the rotor blade 25 may come in contact with theinternal wall of the shroud 55 due to the deformation of the shroud 55,to thereby cause a so-called secondary sliding. In either case, sincethe abrasive particles are firmly brazed at the tip of the rotor blade25, by the coating formation method of the present invention, thecoating of TBC or the like (not shown) formed on the internal wall ofthe shroud 55 can be shaved off. As a result, welding of the rotor blade25 can be prevented, and the gas turbine 100 can be stably operated. Itis preferred that cubic boron nitride is allowed to function withrespect to the initial sliding, and SiC and Al₂O₃ having excellentlong-term stability at high temperatures are allowed to function withrespect to the secondary sliding. Therefore, it is more desirable to mixand use these, for ensuring the long-term reliability of the gasturbine.

[0088] The rotor blade 25 according to the present invention has acoating formed at the tip thereof by brazing, and hence abrasiveparticles such as cubic boron nitride and the like can be distributed ina form of shark teeth. Therefore, even if the abrasive particles on thesurface drop out, the next abrasive particles will appear. Hence, thecoating of TBC or the like formed on the internal wall of the shroud 55can be shaved off stably, until the abrasive coating 23 (see FIG. 3)disappears. As a result, more reliable operation can be realized thanthe case of using a rotor blade, on which a coating is formed by theconventional plating or thermal spraying method.

[0089] The coating formation method of the present invention includes:

[0090] (1) a lamination step of laminating a brazing filler metal layercomposed mainly of a brazing filler metal and a coating material layercomposed mainly of a coating material, on the surface or the back of theobject to be coated;

[0091] (2) a melting step of heating the laminated brazing filler metallayer and the coating material layer to diffuse the coating material andthe brazing filler metal, while allowing the brazing filler metalcomponent to melt and infiltrate in the coating material; and

[0092] (3) a fixing step of solidifying the molten brazing filler metalto fix it on the object to be coated, and a coating is formed by aso-called brazing. This method can be executed at a low cost, ascompared with the plating or thermal spraying method, and does notrequire large-scale equipment. As a result, there is the effect thatthere is little limitation on the application site.

[0093] In the coating formation method according to the presentinvention, the coating parameter between the brazing filler metal andthe coating material laminated at the lamination step is set to from30:70 to 70:30 inclusive. Therefore, not only the brazing filler metalis reliably melted at the melting step, but also the formed coating isfirm, thereby improving the property thereof.

[0094] In the coating formation method according to the presentinvention, since the brazing filler metal contains boron, boron diffusesin the coating material at the melting step, to allow the solidifyingpoint of the coating material to fall. Therefore, even when the coatingmaterial is heated at a relatively low temperature, the coating materialcan be reliably melted, at a low cost.

[0095] In the coating formation method according to the presentinvention, a brazing filler metal having a melting point lower than theheat treatment temperature of the object to be coated is selected. As aresult, the melting step can be executed at the same time with the heattreatment of the object to be coated. Hence, the work efficiency informing the coating can be improved.

[0096] In the coating formation method according to the presentinvention, since the coating material layer, in which coating materialparticles are dispersed in the binder, is used, lamination of thecoating material becomes easy by the binder. The binder substantiallycompletely volatilizes at the melting step, and hence degradation of thecoating resulting from the remaining binder in the coating can besuppressed.

[0097] In the coating formation method according to the presentinvention, the mass ratio between the binder and the coating materialparticles is from 15:85 to 2:1 inclusive. As a result, formation of thecoating material layer becomes easy, and liquid dripping of the brazingfiller metal at the melting step can be suppressed, thereby enablingimprovement of the workability.

[0098] In the coating formation method according to the presentinvention, MCrAlY particles and cubic boron nitride particles are usedas the main component. In the abrasive coating obtained by this coatingmaterial layer, cubic boron nitride serves as abrasive particles, andMCrAlY becomes a matrix to fix the abrasive particles. The MCrAlY matrixcan suppress oxidation of the abrasive particles.

[0099] In the coating formation method according to the presentinvention, the volume ratio between the MCrAlY particles and the cubicboron nitride particles is from 1:2 to 2:1 inclusive. Therefore, thepolishing ability of the abrasive coating is improved and the abrasiveparticles can be reliably fixed.

[0100] In the coating formation method according to the presentinvention, since the abrasive coating is formed at the tip of the rotorblade of a gas turbine, cubic boron nitride in the abrasive coatingpolishes the internal peripheral face of the opposite shroud, and hencea damage of the rotor blade can be prevented.

[0101] In the coating formation method according to the presentinvention, an exposure step of removing a part of MCrAlY from thesurface of the fixed coating material layer to expose the cubic boronnitride particles is included. Further, in the coating formation methodaccording to the present invention, blasting is used at the exposurestep of exposing the cubic boron nitride particles. As a result,exposure of the cubic boron nitride particles can be appropriatelyperformed.

[0102] In the coating formation method according to the presentinvention, in the blasting, an abrasive harder than the MCrAlY particlesbut softer than the abrasive particles is used. As a result, sinceMCrAlY can be removed efficiently from the formed abrasive coating, theabrasive particles can be exposed sufficiently.

[0103] In the coating formation method according to the presentinvention, in the blasting, an abrasive having a particle size smallerthan the particle size of the abrasive particles and smaller than thespaces between the abrasive particles. As a result, since dropout of theabrasive particles can be suppressed to a minimum, while sufficientlyexposing the abrasive particles, sufficient polishing performance can beexhibited from the initial stage.

[0104] In the coating formation method according to the presentinvention, other examples of preferable coating material layers includeone composed mainly of the MCrAlY particles. An oxidation resistantcoating obtained by this coating material layer can be preferably usedin various members of a gas turbine where high-temperature gascirculates, more specifically, in a rotor blade, a stator blade, and ashroud, as in the coating formation method according to the nextinvention.

[0105] In the coating formation method according to the presentinvention, since abrasive particles, a metal material having at least anoxidation resistance, and a binder are contained, the brazing fillermetal is absorbed in the gap produced by volatilization of the binder,in the heat treatment at the time of coating formation. Thereby, liquiddripping of the brazing filler metal can be considerably reduced, andhence the quality after forming the coating on the object to be coatedcan be improved. Further, since the metal material has the oxidationresistance, oxidation hardly occurs even in a high temperaturecombustion gas atmosphere in which the rotor blade of the gas turbine isused. As a result, the abrasive particles can be reliably held, andstable polishing performance can be exhibited even in long-term use, anda reduced thickness due to oxidation of the base metal can be prevented.Hence, more stable operation of the gas turbine can be realized.

[0106] In the coating formation coating material according to thepresent invention, in the coating formation coating material, a ratiobetween the mass of the binder and the mass of the abrasive particlesand the metal material is from 15:85 to 2:1 inclusive. As a result, thecoating material layer can be formed easily, and liquid dripping at themelting step can be suppressed.

[0107] In the coating formation coating material according to thepresent invention, the metal material contained in the coating formationcoating material is MCrAlY. Since MCrAlY having an oxidation resistanceand an intergranular corrosion resistance is used, even when a coatingis formed on the rotor blade of a gas turbine used in a high-temperatureoxidative atmosphere, the abrasive particles can be held for long timeto maintain the polishing performance. As a result, stable operation ofthe gas turbine can be realized.

[0108] In the coating formation coating material according to thepresent invention, in the coating formation coating material, the volumeratio between the MCrAlY particles and the abrasive particles is from1:2 to 2:1 inclusive. Therefore, the occurrence of insufficient brazingfiller metal can be prevented, and the workability can be improved.Further, since abrasive particles can be sufficiently fixed, dropout ofthe abrasive particles can be suppressed, thereby enabling reliableoperation of the gas turbine.

[0109] In the abrasive coating formation sheet according to the presentinvention, a brazing filler metal and any one of the coating formationcoating materials described above are laminated. Therefore, after thisabrasive coating formation sheet is attached to the object to be coated,the abrasive coating can be formed only by heat-treating the object tobe coated. Hence, the abrasive coating can be formed considerablyeasily, as compared with the plating or thermal spraying method.Further, since a treatment prior to the heat treatment is completed onlyby attaching the abrasive coating formation sheet to the object to becoated, the operation becomes very easy. Further, since it is a sheetform, it can be appropriately cut according to the shape of the objectto be coated. Therefore, it can easily correspond to objects to becoated having various shapes.

[0110] In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, the coatingparameter between the brazing filler metal and the coating formationcoating material is set to from 30:70 to 70:30 inclusive. Therefore, notonly the coating formation coating material reliably melts at themelting step, but also the formed coating becomes firm.

[0111] In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, boron is containedin the brazing filler metal. Since boron is contained, this borondiffuses in the coating formation coating material at the melting step,to allow the solidifying point of the coating formation coating materialto fall. Therefore, even when the coating formation coating material isheated at a relatively low temperature, the coating formation coatingmaterial melts. After boron diffuses, since the melting point of thecoating formation coating material increases, the heat resistance of thebrazing filler metal increases. As a result, even when the coatingformation coating material is used in a high-temperature gas, such as inthe rotor blade and the shroud of a gas turbine, the brazing fillermetal can be used without melting.

[0112] In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, the brazing fillermetal is selected from materials having a melting point lower than theheat treatment temperature of the object to be coated. As a result, themelting step is allowed to progress at the same time with the heattreatment of the object to be coated.

[0113] In the abrasive coating formation sheet according to the presentinvention, in the abrasive coating formation sheet, an adhesive layer isformed on the brazing filler metal. Therefore, so long as the abrasivecoating formation sheet is prepared, the treatment prior to the heattreatment is completed only by adhering the abrasive coating formationsheet to the object to be coated, without requiring pasting and waitingfor drying of the paste. As a result, time and energy for coatingformation can be reduced.

[0114] In the rotor blade of a gas turbine according to the presentinvention, an abrasive coating is formed at the tip thereof by any oneof the coating formation methods. Therefore, the abrasive coating can beformed very easily, as compared with the plating or thermal sprayingmethod. As a result, the time required for coating formation can beconsiderably reduced, as compared with the coating formation methoddescribed above, and the production cost thereof can be reduced.

[0115] In the rotor blade of a gas turbine according to the presentinvention, any one of the abrasive coating formation sheets is adheredto the tip thereof. Therefore, the abrasive coating can be formed onlyby performing the necessary heat treatment on the rotor blade, and hencethe abrasive coating can be formed considerably easily, as compared withthe plating or thermal spraying method. As a result, the time requiredfor coating formation can be considerably reduced, as compared with thecoating formation method described above, and the production costthereof can be reduced.

[0116] In the gas turbine according to the present invention, a turbinedriven by a combustion gas injected from the combustor comprises therotor blade. Therefore, if only heat treatment equipment is provided, anabrasive coating can be easily formed on the rotor blade. Hence, evenwhen there is no plating equipment near the operation site of the gasturbine, the abrasive coating can be easily formed, so long as a heatingfurnace used for heat treatment is equipped. As a result, since theabrasive coating can be formed again on the rotor blade on the site,repair of the rotor blade is easy.

INDUSTRIAL APPLICABILITY

[0117] The coating formation method, the coating formation material, theabrasive coating formation sheet, the rotor blade of the gas turbine, onwhich an abrasive coating or the like is formed by the coating formationmethod, and the gas turbine using this rotor blade according to thepresent invention are useful in forming an abrasive coating, anoxidation resistant coating or the like, which is formed on a membersuch as a rotor blade, a stator blade, or a shroud in a combustionengine (gas turbine, jet engine, and the like) and a steam turbine, andis suitable for easily forming these coatings.

1. A coating formation method comprising: a lamination step oflaminating a brazing filler metal layer and a coating material layer onthe surface of an object to be coated, the brazing filler metal layerbeing composed essentially of a brazing filler metal, and the coatingmaterial layer being composed essentially of a coating material; amelting step of heating the laminated brazing filler metal layer and thecoating material layer to allow at least a part of the coating materialto melt, while diffusing the brazing filler metal in the coatingmaterial; and a fixing step of solidifying the melted coating materialto fix it on the object to be coated.
 2. The coating formation methodaccording to claim 1, wherein a coating parameter between the brazingfiller metal and the coating material laminated at the lamination stepis from 30:70 to 70:30 inclusive.
 3. The coating formation methodaccording to claim 1 or 2, wherein the brazing filler metal componentdiffused in the coating material is boron.
 4. The coating formationmethod according to any one of claims 1 to 3, wherein the brazing fillermetal is selected from materials having a melting point lower than theheat treatment temperature of the object to be coated.
 5. The coatingformation method according to any one of claims 1 to 4, wherein thecoating material layer is one in which coating material particlesdiffuse in a binder.
 6. The coating formation method according to claim5, wherein a mass ratio between the binder and the coating materialparticles is from 15:85 to 2:1 inclusive.
 7. The coating formationmethod according to any one of claims 1 to 6, wherein the coatingmaterial layer comprises, as main component, MCrAlY particles andabrasive particles such as cubic boron nitride, Al₂O₃, TiN, or the like.8. The coating formation method according to claim 7, wherein the volumeratio between the MCrAlY particles and the abrasive particles is from1:2 to 2:1 inclusive.
 9. The coating formation method according to claim7 or 8, wherein the object to be coated is a tip of a rotor blade of agas turbine.
 10. The coating formation method according to any one ofclaims 7 to 9, further comprising an exposure step, which is executedafter the fixing step, of removing a part of MCrAlY from a surface ofthe fixed coating material layer to expose the abrasive particles. 11.The coating formation method according to claim 10, wherein the exposurestep is carried out by blasting.
 12. The coating formation methodaccording to claim 11, wherein in the blasting, an abrasive harder thanthe MCrAlY particles but softer than the abrasive particles is used. 13.The coating formation method according to claim 12, wherein the particlesize of the abrasive is smaller than the particle size of the abrasiveparticles and smaller than the space between the abrasive particles. 14.The coating formation method according to any one of claims 1 to 6,wherein the coating material layer is composed essentially of MCrAlY. 15The coating formation method according to claim 14, wherein the objectto be coated is a rotor blade, a stator blade or a shroud of a gasturbine.
 16. A coating material containing abrasive particles such ascubic boron nitride, Al₂O₃, TiN, or the like, a metal material having atleast an oxidation resistance, and a binder.
 17. The coating materialaccording to claim 15 or 16, wherein a ratio between the mass of thebinder and the mass of the abrasive particles and the metal material isfrom 15:85 to 2:1 inclusive.
 18. The coating material according to anyone of claims 15 to 17, wherein the metal material is MCrAlY.
 19. Thecoating material according to of claim 18, wherein the volume ratiobetween the MCrAlY particles and the abrasive particles is from 1:2 to2:1 inclusive.
 20. An abrasive coating formation sheet, wherein abrazing filler metal and a coating formation coating material accordingto any one of claims 15 to 19 are laminated.
 21. The abrasive coatingformation sheet according to claim 20, wherein the coating parameterbetween the brazing filler metal and the coating formation coatingmaterial is from 30:70 to 70:30 inclusive.
 22. The abrasive coatingformation sheet according to claim 20 or 21, wherein boron is containedin the brazing filler metal.
 23. The abrasive coating formation sheetaccording to any one of claims 20 to 22, wherein the brazing fillermetal is selected from materials having a melting point lower than theheat treatment temperature of the object to be coated.
 24. The abrasivecoating formation sheet according to claim 23, wherein an adhesive layeris formed on the brazing filler metal.
 25. A rotor blade of a gasturbine, comprising a coating at the tip thereof, the coating beingformed by the coating formation method according to any one of claims 7to
 13. 26. A rotor blade in a gas turbine, wherein the abrasive coatingformation sheet according to any one of claims 20 to 24 is adhered tothe tip thereof.
 27. A gas turbine comprising: a compressor thatcompresses air to produce combustion air; a combustor that allows thecombustion air produced by the compressor to react with a fuel, togenerate a high-temperature combustion gas; and a turbine having therotor blade according to claim 25, and driven by the combustion gasinjected from the combustor to the rotor blade.